1. Field of the Invention
The present invention relates to an ultrasonic diagnosis apparatus which irradiates an ultrasonic pulse onto a target body under examination, receives an echo pulse from the target body and displays a tomographic image of the body, and in particular, to an improvement of an ultrasonic pulse transmission/ reception system.
2. Description of related art
Conventional major apparatuses for displaying a tomographic image of a target body using ultrasonic pulses are B-mode ultrasonic diagnosis apparatuses as disclosed in "Ultrasonic Pulse Technical Handbook," p 810-813, from Nikkan Kogyo or "Ultrasonic Diagnosis," p 81-84, from Igaku Shoin.
B-mode ultrasonic diagnosis apparatuses are capable of displaying a tomographic image of a sliced section of a target body along the advancing direction of an ultrasonic beam. More specifically, these apparatuses perform a single irradiation of an ultrasonic beam so as to acquire an image for a single scanning line along the advancing direction of the ultrasonic wave, and then perform linear scanning in or sector scanning the irradiating direction of the ultrasonic beam, thereby providing a tomographic image for one screen.
Since ultrasonic pulses generally have a property of gradual diversion with their propagation, the cross-section of each ultrasonic pulse does not become smaller than the size of a probe. For this property, therefore, the B-mode apparatuses have a low azimuth resolution (the resolution in the direction perpendicular to the advancing direction of an ultrasonic beam).
If the tip of the probe is designed to have a recessed surface, the ultrasonic beam would once converge at or near its focal point before it diverges. This ensures that a beam attained is narrower at the proximity of the focal point than the aperture of a transducer, thus improving the azimuth resolution at that region. This method, however, narrows the focusing range (the range within which the diameter of the ultrasonic beam is smaller than a predetermined value) in the depth direction (the advancing direction of the ultrasonic beam), and hence is not practical.
To cope with this problem, therefore, there has been a system in actual use, which uses a transducer with a small aperture for transmission/reception of an ultrasonic wave in such a way as to provide a nearly constant convergence (azimuth resolution) with respect to a certain depth from the skin of the target body and a relatively larger focusing range.
Although the transducer with a small aperture has an advantage of reducing the beam convergence so as to increase the focusing range, it certainly has a disadvantage that the azimuth resolution is deteriorated.
As an alternative, C-mode apparatuses have been proposed, which acquire a number of B-mode tomographic images at given intervals to construct a three-dimensional image, and extracts, from the acquired image, only those pieces of image information which are for the same depth of the target body (preferably, near the focal point) to reconstruct a tomographic image of the target slice along the direction perpendicular to the advancing direction of the ultrasonic beam. This, however, requires a number of B-mode images to acquire a single C-mode image, which is time-consuming and is not practical in actual use in the case where a target slice is in motion.